Sodium percarbonate because of its hygroscopicity to moisture is known to be very unstable compared to sodium perborate tetrahydrate.
Even very small amounts of moisture, as occur for example in washing or cleaning agent mixtures lead to a rapid reduction of the active oxygen content as well as to a clumping of the washing or cleansing agent itself.
Therefore, it has already been tried to counteract this disadvantage in the use of sodium percarbonate, namely by addition of known stabilizers for hydrogen peroxide, e.g. alkaline earth silicates (Meier-Evert, German Pat. No. 870,092.
Even if silica containing compounds, namely silica itself, are added subsequently in preference to other materials for stabilization (as coatings of paraffin or polyethylene glycol or to the use of several stabilizers simultaneously (German OS No. 2511143) the stability of the percarbonate attained therewith in moist atmosphere was still very insufficient.
The reason is first because of industrially incompletely carrying out the contacting of the percarbonate and the stabilizing silica containing compound.
Thus if there were added for example silica aerosols (which themselves are very difficult to disperse) during the production of the percarbonate to the reaction mixture or to the dry percarbonate powder, or in certain cases even to the already prepared percarbonate solution (German Pat. No. 870,092 and German OS No. 2448453) the two materials present subsequently separate from each other and a stabilizing effect can scarcely occur.
Or the encasing of the percarbonate particles can take place. However, the necessary industrial procedure is extremely troublesome and besides does not always lead to a sufficient encasing.
Thus according to the process of German OS No. 2448453 sodium percarbonate was sprayed with an aqueous sol which contains 3-8% silica and has a pH of 3 to about 10.
However, this encasing material was produced by deionization of water soluble silicates such as alkali silicates with the help of cation exchangers. The aqueous sols thus produced, however, are not stable over long periods of time, but must be further used in a suitable time. In other words, the stabilization of the percarbonate continuously requires an operationally ready ion exchange equipment, since the thus produced sols are not stable.
In spite of this expense after only 6 days in the washing agent storage test in moist atmosphere in the most favorable case there was already destroyed 10% of the active oxygen.
Since coatings of alkali silicate are hygroscopic because of their reaction with the carbon dioxide content of the air, there were used in German OS 2511143 aqueous mixtures of alkali or alkaline earth silicates and similarly silicofluorides for coating the percarbonate particles. These particles are present in the dry condition and are sprayed with the above mentioned mixture.
A spraying of silica containing sols, however, is not always simple to carry out because of the danger of crystallization and requires a specific standard industrial expense. Therefore, both of the last mentioned processes of the German OS Nos. 2448453 and 2511143 are quite expensive industrially.
The object of the invention is to provide sodium percarbonate in a simple manner with a silica containing coating and to obtain a product stable against moisture.